1. Field of the Invention
The present invention relates to a liquid discharge head incorporating a unimorph type piezoelectric element using a piezoelectric thin film (piezoelectric film) and a method of manufacturing thereof, and also a method of manufacturing a piezoelectric element. The present invention can apply to any of various devices using a driving force of a piezoelectric element, including a liquid discharge head incorporated in a recording apparatus such as a printer.
2. Description of the Related Art
These years, the studies of devices using functional thin films have been prosperous, and it has been expected to materialize excellent functions by forming a functional material into a thin-film which is incorporated in any of various devices.
For example, studies of devices including piezoelectric elements, sensors, nonvolatile memories and the like, using physical properties such as piezoelectricity, pyroelectricity, polarization reversal have been prosperous. Among others, recording apparatuses of liquid discharge type in which liquid such as ink is discharged by a piezoelectric driving force have been rapidly developed since it can record an image having a highly precise and fine quality and a high density at a high speed, and since it can be appropriate for color printing, and compact so as to be applied in not only printers but also copiers, facsimiles and the like. In such a technical field of recording, there has been increased such a demand that the recording technology enhances the recording quality and the degree of recording accuracy in future. As one of various ways for materializing the demand, a piezoelectric element utilizing a piezoelectric thin film (piezoelectric film) is used, and is expected to be applied in a high quality and high precise recording technology for the next generation.
There can be enumerated various methods of manufacturing piezoelectric films. For example, Japanese Patent Laid-Open No. H06-290983 discloses a film forming method for a PZT film, using RF sputtering. Further, Japanese Patent Laid-Open No. H11-220185 discloses a method of forming a PZT film oriented in (100) plane under control of precursor decomposing temperature in a sol-gel process.
There can be enumerated various types of piezoelectric element utilizing a piezoelectric film. Among others, a unimorph type piezoelectric element in which a diaphragm having a Young's modulus different from that of a piezoelectric material, is laminated thereover with a piezoelectric film is extremely excellent, and accordingly, it can be simply applied to a liquid discharge head.
As one of the liquid discharge heads using the unimorph type piezoelectric element as a drive source, there may be exemplified the one having such a configuration that a glass substrate (glass diaphragm) which is anodically joined to an Si substrate as a passage substrate is transferred thereonto with a piezoelectric film deposited on another substrate. Since the glass substrate serves as an excellent diaphragm, and has a linear expansion coefficient which is nearly equal to that of the Si substrate, it is appropriate to anodically join the glass substrate onto the Si substrate in order to form a unimorph type piezoelectric element.
Almost functional thin films are oxides, and in particular, a thin film having piezoelectricity is in general a composite oxide, and accordingly, the crystallization thereof requires a high temperature. For example, a high temperature of not less than about 1,000° C. is required for crystallization of a bulk body of a piezoelectric material, and further, a high temperature of substantially 800 to 900° C. is required for crystallization of even a thin film with the use of annealing in, for example, a sol-gel process. Accordingly, for the crystallization, there has been used such a method that a thin film is deposited on an additional substrate with no heat, and after the deposition of the film, the film is annealed, or a method that an additional substrate is heated for crystallization while a piezoelectric film is deposited. However, since the crystallization requires a high temperature, a single crystal substrate which can resist the high temperature is required for the additional substrate on which a piezoelectric film is deposited. There may be enumerated, as typical one, MgO, SrTiO3 and the like, which are extremely expensive in general. Thus, it is relatively disadvantageous to use these materials for the additional substrate which is consumed away by one time of film deposition.
In addition, in the case of deposition of a piezoelectric film on a single crystal substrate, since only the single crystal substrate should be removed by melting with hot phosphoric acid or the like after it is bonded to a glass substrate serving as a diaphragm, and since this melting requires a very long time, it is extremely disadvantageous in view of not only the costs but also the throughput thereof, resulting in a great barrier against the mass-production thereof.
In order to solve the above-mentioned problems, it is effective to use a method of depositing a piezoelectric film on a glass substrate serving as a diaphragm, direct thereto. For example, Japanese Patent Laid-Open No. H07-246705 discloses a method of depositing a PZT film, direct onto SiN sputtered onto an Si substrate through the intermediary of a zirconia film as a lead diffusion preventing layer. However, the linear expansion coefficient of SiN is extremely small in comparison with that of Si, and accordingly, the PZT film is susceptible to peel off from the Si substrate during a heat-treatment process, that is, it is disadvantages in view its process. Further, even though the heat-treatment can be completed without peel-off, it is thereafter required to etch the rear surface of the Si substrate in order to form flow passages including a pressure generation chamber, and further to be mated with a liquid supply system for ink or the like, which has been separately formed. In this case, a loss is possibly caused during bonding between the finely processed articles, and accordingly, there would be caused a risk of lowering the yield thereof. That is, it is difficult to enhance the yield since the Si substrate cannot be processed beforehand.
Further, Japanese Laid-Open Patent No. H07-246705 discloses a method in which a glass substrate incorporating ITO electrodes and serving as a diaphragm is anodically joined to a head base formed therein with flow passages, and screen printing of PZT octylate chloride is calcined and crystallized at a temperature of 500° C. However, the diaphragm has a thickness of not less than several tens of micron meter so as to be able to be handled, and accordingly, the joint part of the diaphragm is directly subjected to affection of thermal strain caused by a difference in thermal expansion during heat-treatment for crystallization. Thus, there is a risk of lowering the joint strength during the heat treatment, and further, it is difficult to completely crystallize the PZT base at a temperature of 500° C.
Further, Japanese Patent Laid-Open No. H05-286132 discloses a method in which a glass ceramic substrate serving as a diaphragm is anodially joined to a head base formed therein with flow passages, and screen printing of a PTZ paste is calcined and crystallized at a temperature of 1,000° C. In this case, since the diaphragm also requires a thickness of not less than several tens of micron meter so as to be able to be handled, and in addition, since the joint part of the diaphragm is susceptible to thermal strain caused by a difference in thermal expansion, the joint strength is deteriorated, and further, the glass ceramic substrate can hardly resist against a thigh temperature of 1,000° C. for crystallization of the PZT. Further, it cannot be assured to prevent the joint part of the diaphragm by anodic joint from peeling off at a high temperature up to 1,000° C.
By the way, such a method that a piezoelectric film is directly deposited on a heat resistant diaphragm without using a transfer process is also effective. As to the method in which the piezoelectric film is directly deposited on the heat resistant diaphragm, as disclosed in Japanese Laid-Open Patent No. 2000-52550, there is a method in which the surface of an Si substrate is thermally oxidized so as to form an SiO2 layer, and it is used as a diaphragm.
However, in the case of such a technique that a piezoelectric film is formed on a diaphragm with no use of a transfer process, there may be enumerated the following points to be improved: In the configuration disclosed in Japanese Patent Laid-Open No. 2000-52550, a PZT film is directly deposited on an SiO2 layer formed on an Si substrate, and is then crystallized, and thereafter, the Si substrate is cut out by etching, at a surface on the side remote from the PTZ film, so as to form flow passages including a pressure generation chamber. In such a manufacturing method, when the PZT film is cooled after it is crystallized at a high temperature, the lattice constant thereof is greatly changed being affected by a thermal expansion coefficient of the Si substrate serving as a film deposition substrate, and accordingly, the piezoelectricity of the PZT film is greatly deteriorated. Although the reason why this phenomenon is caused cannot completely be clarified, there may be considered the following points:
Although the thermal expansion coefficient of the PTZ film varies, depending upon its composition, around an MPB composition (Zr:Ti=0.53:0.47) having a highest piezoelectricity, it is about 9×10−6 (/° C.). Meanwhile, the thermal expansion coefficient of the Si substrate is 3×10−6 (/° C.) which is relatively lower than that of the PZT film. Thus, when the PZT film is cooled to a room temperature by way of a Curie point after it is crystallized, the PZT film greatly contracts, but the degree of contraction of the Si substrate is small, and accordingly, the PZT film is subjected to a large force in a tensioning direction. In order to relax this force, the orientation of the PZT crystal which is tetragonal is mostly directed in the in-plane direction of the Si substrate in which C-axis having a long crystalline axis is subjected to a tension force. Since the polarizing axis of the PZT film which is tetragonal is in the C-axial direction, the crystalline in which the polarizing direction is vertical, that is, the so-called 90 degree domain, is dominative, with respect to the vertical direction of the substrate plane to which an electric field is applied. Thus, it may be considered that the piezoelectricity is possibly deteriorated by a large degree.
Meanwhile, Japanese Patent Laid-Open No. 2000-141644 discloses such a configuration that an intermediate film is provided for applying tensile stress to a PZT film which is formed on an Si substrate formed thereon with a SiO2 layer serving as a diaphragm. The reason why the intermediate film is provided is such as to prevent occurrence of such a risk that since the thermal expansion coefficient of the PZT film is greater than that of the SiO2 layer, when flow passages including a pressure generation chamber is formed on the side remote from the PZT film which is a piezoelectric film, the SiO2 diaphragm having a thin thickness of several micron meters, is subjected to a force in the direction of compression due to a difference in thermal expansion with respect to the PZT film, and accordingly, it is deformed toward the liquid flow passage. However, in this method, the 90 deg. domain which does not contribute to the piezoelectricity of the PZT film, tends to contrarily increase, and accordingly, the piezoelectricity is remarkably deteriorated.
Further, Japanese Patent Laid-Open No. H07-246705 discloses a method in which a PZT film is deposited on an SiN layer sputtered on an Si substrate and serving as a diaphragm, through the intermediary of a zirconia film for preventing diffusion of lead. Since the thermal expansion coefficient of the zirconia film is greater than that of the PZT film, the provision of such a film between the diaphragm and the piezoelectric film is effective for decreasing tensile stress to the piezoelectric film even though its purpose is different more or less. However, since the stress is the product of a Young modulus and a degree of strain, a stress caused by heat hysteresis is proportional to a product of the thermal expansion coefficient of its material and its Young modulus. Accordingly, Since the lengths with which the films make contact with each other are equal to each other among the areas, the film thickness is problematic, and accordingly, if a specific relationship cannot not satisfied, the tensile stress applied to the PZT film cannot be reduced.